When this flag is set to t, subterms that occur more than once in
a clause are abstracted away with let*, generally shortening
the displayed size of the clauses. This flag only affects how
clauses are printed. It does not change what terms the theorem
prover manipulates.

:set-let*-abstractionp t ;;; or, (set-let*-abstractionp t)

will cause the prettyprinter to do ``let* abstraction'' on clauses
before they are printed. The algorithm finds the maximal
multiply-occuring subterm and extracts it, binding it to some new
variable and replacing its occurrences by that variable. This produces
a let* form. This process is iterated until no subterm occurs more
than once. This process generally takes a little time, but less time
than to print large clauses. The process can greatly reduce the amount of
text produced by the prover.

THIS ONLY AFFECTS HOW THE CLAUSES ARE PRINTED! The unabstracted
clauses are manipulated by the theorem prover.

:set-let*-abstractionp nil

restores normal clause printing.

The mode is stored in the defaults table, See acl2-defaults-table.
Thus, the mode may be set locally in books.